1. Field
The following description relates to Radio Frequency Identification (RFID), and more particularly, to a synchronization technology for modulating a signal input to a tag.
2. Description of the Related Art
The International Organization for Standardization/International Electrotechnical Commission (ISO/IEC) 18000 series details international standards for Radio Frequency Identification (RFID) wireless access, and 13.56 MHz standards are defined in ISO/IEC 18000 Part 3. Part 3 is classified into mode-1, mode-2, and mode-3 according to a modulation scheme and an encoding scheme.
Mode-1 has been suggested by Philips and Texas Instruments (TI) and adopts an amplitude shift keying (ASK) modulation scheme and a pulse position modulation (PPM) encoding scheme. It is a communication standard that uses a single channel. Mode-2 has been suggested by Magellan Technology and adopts a phase jitter modulation (PJM) scheme and a double frequency modified frequency modulation (DFMFM) encoding scheme. It is a communication standard that uses multiple channels. Mode-3 can adopt both of the ASK and PJM modulation schemes and thus uses mode-1 and mode-2 together. An ASK mode adopts pulse interval encoding (PIE) as an encoding scheme, and a PJM mode adopts modified frequency modulation (MFM) as an encoding scheme. Mode-3 is intentionally made to have the same logical layer as 18000-6 Type C which is an RFID communication standard for an ultra-high frequency band. Thus, mode-3 lays the foundation for technological advancement toward the integration of an ultra-high frequency band and a high frequency band.
Since the PJM mode of mode-3 is for communication using multiple channels, it has a far shorter tag recognition time than the ASK mode using a single channel. However, it is impossible to detect an envelope of a signal input to a tag, which makes it difficult to extract a payload data start position. In addition, since a phase difference between two PJM signals is very small, it is difficult to demodulate the phases thereof.
A passive PJM tag is charged with power in a continuous wave (CW) section. When receiving a command from a reader, the passive PJM tag must recognize the command. In addition, the passive PJM tag must identify a position where an MFM flag, which is a preamble to the command, ends and demodulate payload data that follows the MFM flag. However, it is not easy for the tag to recognize a change in a signal, which is received from the reader, when it changes from a CW to a command and determine a start position of payload data, which is to be demodulated, even when it has been recognized that the command is being received. Furthermore, since a phase difference between PJM signals is very small, it is not easy to demodulate the PJM signals.